The international group Doctors Without Borders warned Tuesday that the world is losing the battle against Ebola and lamented that treatment centers in West Africa have been “reduced to places where people go to die alone.”

In separate remarks after a United Nations meeting on the crisis, the World Health Organization chief said everyone involved had underestimated the outbreak, which has now killed more than 1,500 people in Guinea, Liberia, Sierra Leone and Nigeria. U.N. officials implored governments worldwide to send medical workers and material contributions.

Meanwhile in Liberia, a missionary organization announced that another American doctor has become infected.

Doctors Without Borders, which has treated more than 1,000 Ebola patients in West Africa since March, is completely overwhelmed by the disease, said Joanne Liu, the organization’s president. She called on other countries to contribute civilian and military medical personnel familiar with biological disasters.

“Six months into the worst Ebola epidemic in history, the world is losing the battle to contain it,” Liu said at a U.N. forum on the outbreak. “Ebola treatment centers are reduced to places where people go to die alone, where little more than palliative care is offered.”

In Sierra Leone, she said, infectious bodies are rotting in the streets. Liberia had to build a new crematorium instead of new Ebola care centers.

At the U.N. meeting, WHO Director Margaret Chan thanked countries that have helped but said: “We need more from you. And we also need those countries that have not come on board.”

Later at a news conference, she warned that the outbreak will get worse before it gets better.

President Barack Obama urged West Africans on Tuesday to wear gloves and masks when caring for Ebola patients or burying anyone who died of the disease. He discouraged the burial practice of directly touching the body of Ebola victims, which is one way the disease has been spreading.

“You can respect your traditions and honor your loved ones without risking the lives of the living,” Obama said in the brief video message.

Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said the situation is now the world’s first Ebola epidemic, given how widely it is spreading.

The latest missionary to come down with the disease, a male obstetrician, was not immediately identified by the group Serving In Mission. The group did not specify how he contracted the disease, but it can be spread through vaginal fluids. He did not work in an Ebola ward.

A Liberian doctor on the missionary’s treatment team said it was too soon to tell whether he will be evacuated. The doctor would speak only on condition of anonymity because he was not authorized to discuss the matter with reporters.

Last month, two Americans, including one from the same missionary group, were evacuated to the United States for treatment after contracting Ebola in Liberia. The two recovered after receiving an experimental drug known as ZMapp. The manufacturer says it has run out of supplies of the drug and it will take months to produce more.

U.S. health officials on Tuesday announced a $24.9 million, 18-month contract with Mapp Biopharmaceutical Inc. to speed development of ZMapp. As part of the project, Mapp is to make a small amount of the drug for early-stage safety testing, while working with the Department of Health and Human Services to accelerate the manufacturing process.

The outbreak has taken a particularly high toll on health care workers, and nurses in Liberia and Sierra Leone have repeatedly gone on strike to demand hazard pay and better protective gear.

On Monday, nurses at a major hospital in the Liberian capital went on strike, according to spokesman Jerald P. Dennis III. While JFK hospital is treating Ebola patients, the striking nurses were all from non-Ebola wards.

Information Minister Lewis Brown said late Tuesday that the dispute had been resolved, but Dennis said discussions were ongoing.

Meanwhile, the Sierra Leone government said nurses were back at work Tuesday after a strike at a Freetown hospital this weekend. The government has said it will pay out all accrued hazard pay and double the allowance going forward.

Also Tuesday, the U.N. Food and Agriculture Organization warned that food in countries hit by Ebola is becoming more expensive and will become scarcer because some farmers can’t reach their fields.

Authorities have cordoned off entire towns in an effort to halt the virus’ spread. Surrounding countries have closed land borders, and airlines have suspended flights to and from the affected countries. Seaports are losing traffic, restricting food imports to the hardest-hit countries.

Those countries — Guinea, Liberia and Sierra Leone — all rely on grain from abroad to feed their people, according to the U.N. food agency. For instance, the price of cassava root, a staple in many West African diets, has gone up 150 percent in one market in Liberia’s capital, Monrovia.

“Even prior to the Ebola outbreak, households in some of the affected areas were spending up to 80 percent of their incomes on food,” said Vincent Martin, who is coordinating the food agency’s response to the crisis. “Now these latest price spikes are effectively putting food completely out of their reach.”